1. Technical Field
This invention relates to a wheel assembly that can traverse rough surfaces and roll over small obstacles without dragging.
More particularly, this invention relates to a wheel assembly which comprises a wheel holder that is rotatable about a central axis, the wheel holder having mounted thereon at least four independently rotatable wheels.
2. Background Art
Hand trucks or dollies used to transport loads up and down stairways have long used a wheel arrangement consisting of a pair of star-shaped plates which rotate about a main axle and include a number of wheels, usually three or four, mounted on stub axles. Such a wheel arrangement is often referred to as a spider or star wheel. The plates and wheels are sized such that two of the stub axle mounted wheels can be in simultaneous contact with the lands of two adjacent steps.
A stair climbing hand truck equipped with a pair of star wheels, each having four radially projecting arms spaced at 90xc2x0 one to another, is described in U.S. Pat. No. 2,706,640 to Marshall. Roller wheels are mounted at the arm ends. The star wheel used by Marshall is sized such that two adjacent roller wheels can simultaneously contact the lands of two adjacent steps when the hand truck ascends or descends a stairway.
Another hand truck having stair climbing capabilities is disclosed in the Boyd patent, U.S. Pat. No. 4,142,732. The Boyd truck employs a pair of star-like plates journaled on a main axle with each plate carrying three stub axles, one on each projecting plate leg. A wheel is mounted on each stub axle and the plate and wheels are sized such that wheels are in contact with the lands of two adjacent steps at all times while the truck is on a stairway.
A motorized wheel chair that is able to go up and down stairs and climb sidewalk or pavement curbs is shown in U.S. Pat. No. 4,709,772. The wheel chair uses a pair of wheel assemblies, each comprising three wheels mounted on a triangular support frame. Each frame and wheel assembly is rotated by a powered shaft that is fixed to the frame center.
Wheel assemblies for traversing obstacles and to climb stairs are shown in U.S. Pat. No. 3,326,563 to Whitacker, U.S. Pat. No. 4,457,526 to Persson, and U.S. Pat. No. 4,687,213 to Ridderstolpe. The wheel assemblies described in the three patents all use a star or spider wheel arrangement in which three wheels are mounted symmetrically on a plate or support member which in turn rotates about a main shaft or axle.
While the wheel assemblies of the type shown in the above listed prior art work well at climbing stairs and curbs, none of those designs can be successfully scaled down in size to use on items which require a relatively small diameter wheel such as luggage, in-line skates and skate boards, grocery carts, electronic test equipment carts and the like. The need for a simple and inexpensive wheel assembly that can traverse small obstructions such as hose lines, electrical cords, twigs and small branches, gravel, sidewalk irregularities and the like without wheel dragging has long been present. This invention fills that need.
This invention provides a wheel assembly that can freely roll or climb over small obstacles such as electrical cords, pavement irregularities, small stones and other trash without wheel drag. The assembly includes a primary wheel member that is rotatable about a central axis. At least four smaller secondary wheels are mounted on axles attached around the circumference of the primary wheel at locations equidistant from each other and from the central axis. Each secondary wheel has a diameter that is greater than is the distance between adjacent wheel locations, and is free to rotate about its axis. The wheel assemblies of this invention roll easily across electrical cords, pneumatic lines and small trash, and are particularly useful on rolling luggage, tool carts, electronic test equipment, skates, chairs, and the like.
Thus, the primary object of this invention is to provide a wheel assembly that rolls over obstacles without wheel drag. Other objects of the invention will be evident from the drawings and the description of preferred embodiments.